Light olefins, such as ethylene, propylene, butylenes and mixtures thereof, serve as feeds for the production of numerous important chemicals and polymers. Typically, C2–C4 light olefins are produced by cracking petroleum refinery streams, such as C3+ paraffinic feeds. In view of limited supply of competitive petroleum feeds, production of low cost light olefins from petroleum feeds is subject to waning supplies. Efforts to develop light olefin production technologies based on alternative feeds have therefore increased.
An important type of alternative feed for the production of light olefins is oxygenates, such as C1–C4 alkanols, especially methanol and ethanol; C2–C4 dialkyl ethers, especially dimethyl ether (DME), methyl ethyl ether and diethyl ether; dimethyl carbonate and methyl formate, and mixtures thereof. Many of these oxygenates may be produced from alternative sources by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastic, municipal waste, or any organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates have promise as economical, non-petroleum sources for light olefin production.
The preferred process for converting an oxygenate feedstock, such as methanol, into one or more olefin(s), primarily ethylene and/or propylene, involves contacting the feedstock with a crystalline molecular sieve catalyst composition. Crystalline molecular sieves have a 3-dimensional, four-connected framework structure of corner-sharing [TO4] tetrahedra, where T is any tetrahedrally coordinated cation. Among the known forms of molecular sieve are aluminosilicates, which contain a three-dimensional microporous crystal framework structure of [SiO4] and [AlO4] corner sharing tetrahedral units silicoaluminophosphates (SAPOs), in which the framework structure is composed of [SiO4], [AlO4] and [PO4] corner sharing tetrahedral units.
Molecular sieves have been classified by the Structure Commission of the International Zeolite Association according to the rules of the IUPAC Commission on Zeolite Nomenclature. According to this classification, framework-type zeolite and zeolite-type molecular sieves, for which a structure has been established, are assigned a three letter code and are described in the Atlas of Zeolite Framework Types, 5th edition, Elsevier, London, England (2001), which is herein fully incorporated by reference.
Among the molecular sieves that have been investigated for use as oxygenate conversion catalysts, materials having the framework type of the zeolitic mineral chabazite (CHA) have shown particular promise. For example, SAPO-34 is a crystalline silicoaluminophosphate molecular sieve of the CHA framework type and has been found to exhibit relatively high product selectivity to ethylene and propylene, and low product selectivity to paraffins and olefins with four or more carbon atoms.
The preparation and characterization of SAPO-34 have been reported in several publications, including U.S. Pat. No. 4,440,871; J. Chen et al. in “Studies in Surface Science and Catalysis”, Vol. 84, pp. 1731–1738; U.S. Pat. No. 5,279,810; J. Chen et al. in “Journal of Physical Chemistry”, Vol. 98, pp. 10216–10224 (1994); J. Chen et al. in “Catalysis Letters”, Vol. 28, pp. 241–248 (1994); A. M. Prakash et al. in “Journal of the Chemical Society, Faraday Transactions” Vol. 90(15), pp. 2291–2296 (1994); Yan Xu et al. in “Journal of the Chemical Society, Faraday Transactions” Vol. 86(2), pp. 425–429 (1990).
According to the article entitled “Identification of a Key Precursor Phase for Synthesis of SAPO-34 and Kinetics of Formation Investigated by In Situ X-ray Diffraction” by O. B. Vistad et al., J. Phys. Chem. B, 2001, 105, pages 12437–12447, the synthesis of SAPO-34 in the presence of HF and with morpholine as a structure directing agent proceeds through the formation of a layered crystalline precursor phase. The layered precursor is reported to be formed at synthesis temperatures of 90° C. to 150° C., whereas SAPO-34 is formed at temperatures of 170° C. to 210° C.
Similarly, the article entitled “Synthesis of AlPO4-11” by N. J. Tapp et al., Zeolites, 1988, Vol. 8, 183–188 discloses that the synthesis of AlPO4-11 free of condensed phase impurities is aided by pretreatment of the synthesis gel at 90° C. The pretreatment is reported to produce a poorly crystalline metavariscite/variscite phase that is transformed to AlPO4-11 on raising the temperature to 200° C.
Regular crystalline molecular sieves, such as the CHA framework type materials, are built from structurally invariant building units, called Periodic Building Units, and are periodically ordered in three dimensions. Disordered structures showing periodic ordering in less than three dimensions are, however, also known. One such disordered structure is a disordered planar intergrowth in which the building units from more than one framework type, e.g., both AEI and CHA, are present. One well-known method for characterizing crystalline materials with planar faults is DIFFaX, a computer program based on a mathematical model for calculating intensities from crystals containing planar faults (see M. M. J. Tracey et al., Proceedings of the Royal Chemical Society, London, A [1991], Vol. 433, pp. 499–520).
International Patent Publication No. WO 02/70407, published Sep. 12, 2002 and incorporated herein by reference, discloses a silicoaluminophosphate molecular sieve, now designated EMM-2, comprising at least one intergrown form of molecular sieves having AEI and CHA framework types, wherein said intergrown form has an AEI/CHA ratio of from about 5/95 to 40/60 as determined by DIFFaX analysis, using the powder X-ray diffraction pattern of a calcined sample of said silicoaluminophosphate molecular sieve. EMM-2 has been found to exhibit significant activity and selectivity as a catalyst for the production of light olefins from methanol (MTO).
According to International Patent Publication No. WO 02/70407, EMM-2 can be synthesized by mixing reactive sources of silicon, phosphorus and a hydrated aluminum oxide in the presence of an organic directing agent, particularly a tetraethylammonium compound. The resultant mixture is stirred and heated to a crystallization temperature, preferably from 150° C. to 185° C., and then maintained at this temperature under stirring for between 2 and 150 hours.
U.S. Pat. No. 6,334,994, incorporated herein by reference, discloses a silicoaluminophosphate molecular sieve, referred to as RUW-19, which is also said to be an AEI/CHA mixed phase composition. In particular, RUW-19 is reported as having peaks characteristic of both AEI and CHA framework type molecular sieves, except that the broad feature centered at about 16.9 (2θ) in RUW-19 replaces the pair of reflections centered at about 17.0 (2θ) in AEI materials and RUW-19 does not have the reflections associated with CHA materials centered at 2θ values of 17.8 and 24.8. DIFFaX analysis of the X-ray diffraction pattern of RUW-19 as produced in Examples 1, 2 and 3 of U.S. Pat. No. 6,334,994 indicates that these materials are characterized by single intergrown forms of AEI and CHA framework type molecular sieves with AEI/CHA ratios of about 60/40, 65/35 and 70/30.
According to the '994 patent, RUW-19 can be synthesized by initially mixing an Al-source, particularly Al-isopropoxide, with water and a P-source, particularly phosphoric acid, and thereafter adding a Si-source, particularly colloidal silica and an organic template material, particularly tetraethylammonium hydroxide, to produce a precursor gel. The gel is then put into a steel autoclave and, after an aging period at room temperature, the autoclave is heated to a maximum temperature between 180° C. and 260° C., preferably at least 200° C., for at least 1 hour, with the autoclave being shaken, stirred or rotated during the entire process of aging and crystallization. Factors which are said to enhance the production of the mixed phase RUW-19 material include maintaining the SiO2 content of the gel below 5%, reducing the liquid content of the gel after addition of the SiO2 source and crystallization at temperatures of 250° C. to 260° C. Pure AEI and CHA phases are said to be favored at temperatures of 200° C. to 230° C.
Study of the synthesis of EMM-2 has now shown that the crystallization process to produce such AEI/CHA intergrowths proceeds through the formation of a (silico)aluminophosphate hydrate precursor, such as ALPO-H3 and/or variscite and/or metavariscite, during heat-up of the mixture, followed by dissolution of the precursor as the intergrown molecular sieve nucleates. Moreover, it has been found that optimal conditions for the formation of the precursor are different from the optimal conditions for conversion of the precursor to the intergrown molecular sieve. For example, whereas agitation of the synthesis mixture seems to be important in initial precursor formation, synthesis of the intergrown molecular sieve from the precursor slurry can proceed with no or reduced agitation. In addition, the presence of an organic directing agent seems to be more important during nucleation of EMM-2 than during precursor formation. In fact, the absence of an organic directing agent seems to allow the precursor to crystallize under more mild conditions (for example at lower temperature).
Accordingly, the present invention provides a method of synthesizing a silicoaluminophosphate molecular sieve comprising at least one intergrown phase of an AEI framework type and a CHA framework type, in which the precursor formation stage and the molecular sieve nucleation stage are decoupled whereby each stage can be conducted under the most advantageous conditions.